صور الصفحة
PDF
النشر الإلكتروني
[graphic][subsumed][subsumed][subsumed][merged small][subsumed][subsumed][subsumed][subsumed][subsumed][subsumed][subsumed][subsumed][subsumed][subsumed][subsumed][subsumed][subsumed][merged small]

CHART SHOWING COMPARATIVE NUMBER OF BACTERIA FOUND IN ONE CUBIC CENTIMETER OF POTOMAC WATER IN 1886. ALSO DEATH RATE

OF TYPHOID FEVER IN 1886.

By Dr. Theobald Smith.

and the relation it bears to the monthly death rate of typhoid fever. It is an established fact that during the summer time the number of bacteria is diminished by certain agencies which will be spoken of later. It has been found that there is

a relative increase in the number of intestinal and sewage bacteria during this time. This fact alone, it is believed, will go far in establishing the connection between typhoid fever and this group of bacteria.

There is a wide difference of opinion concerning the viability of bacteria in water. Some claim that they will not develop in water unless it is very rich in nutrient material, and then only under the most favorable conditions, while there are others who cite instances where the bacteria material increases under the ordinary conditions, especially those of the intestinal group.

It is also a well-recognized fact that temperature plays an important rôle in the development of bacteria, and we would expect a large increase in their number during the summer and fall. Practically this does not occur.

Water containing sewage will preserve bacteria for a considerable length of time, especially if the temperature is low. The bacteria in water may be diminished by the germicidal action of strong sunlight, especially if the water is clear and still; yet under the most favorable conditions the sunlight affects only the under stratum, and has scarcely any effect upon those a foot or so below the surface.

There is little or no effect if the water is turbid or moving rapidly.

It requires a long time for bacteria to be affected by aeration. They will diminish under its influence if it is persistent, but not otherwise. The inference drawn from the oxidation of the bacterial and chemical products does not apply to the bacteria. The number of bacteria in the Niagara River are but slightly diminished in their plunge over the falls and subsequent aeration in the stream below. Perhaps the stream is not long enough to test this thoroughly, but I am of the opinion that there are no rivers in this country of sufficient length to accomplish this purpose.

There is no doubt but that sedimentation accomplishes considerable in diminishing the number of bacteria. Sedimentation of bacteria is quite another thing from the deposition of suspended matter. A stream may free itself from turbidity and yet contain a large number of bacteria. Sedimentation of bacteria may, and often does, occur as the result of time and the distance the water is carried. The slower the volume of water is moved the more rapid will be the deposition. It remains to be demonstrated whether any sedimentation occurs in the Potomac River above Washington. If we take the turbidity as an index the sedimentation must be slight. So far the number of observations have not been sufficient to settle this question.

A series of observations were recently made of the water in the receiving reservoir, which is located a short distance above the city. These observations were to determine the number of bacteria at different depths in the reservoir. The following are the results:

[blocks in formation]

It was contrary to expectation to find the number of bacteria greater on the surface than at points below. This is explained by the rapid current of the water passing from one reservoir into the other and out at the gate, thereby practically making a stream through the reservoir. The water evidently flows in the line of least resistance, and this is found at the surface. Consequently, the bulk of water passes through the reservoir in a surface current and allows the under stratum to deposit its bacteria. It also points to the fact that sedimentation does occur, but to a slight degree, especially so in those localities where the current is slackened or absent. It is not believed that the short time of three days would be sufficient to influence materially the number of bacteria.

I am informed that the reservoir contains 150,850,000 gallons, which is a three days' supply for the city.

Observations which were conducted with the London water supplyduring 1896 show that it requires at least ten days' storage before sedimentation diminishes the number of bacteria, and if the water was allowed to remain in storage for a month or more the sedimentation was more complete.

The same conditions will apply to the public water supply. I am unable to state whether the Delecarlia and receiving reservoirs are of sufficient size to permit the storage of water for this length of time.

There is an intimate relation existing between the number of bacteria present in a water supply and disease. Rivers or other sources of water supply which contain a large number of bacteria are responsible for the large number of diarrheal disorders. So if the number of bacteria is high and constant a water should always be looked upon with suspicion. A sudden rise in the number of bacteria in the Potomac water may not be of great importance, but the persistence of this large number is always to be regarded with suspicion. It is to be expected that a water supply receiving large quantities of decomposing organic matter-sewage, in other words-has associated with it a great number of bacteria. There is always a relative increase in the number of bacteria in river water during winter. This is due to two causes: First, increased washings from the soil; and second, the preservative action of low temperatures.

It will be observed that during these months the typhoid-fever death rate is at its minimum, notwithstanding the increased number of micro-organisms. At first sight one might conclude that this would be a contradiction of the foregoing statement. The increase in the number of bacteria is synchronous with the increased volume, notably just after rains or during the winter months, when the precipitation is the heaviest. It would be natural to infer that if the number of bacteria were indicative of contamination there should be an increase in the number of diarrheal diseases. Yet this is not the rule. There are, however, exceptions. During the summer months there is an occasional heavy rain, which carries the sewage from the watershed to the river, and this rain is not sufficient to materially augment the body of water in the river. In this way it is probable that large quantities of concentrated sewage may suddenly be thrown into the water and do great harm. In the summer time the stream and tributaries receive a given quantity of sewage daily, the water becoming less in volume, thereby increasing the number of sewage bacteria many times over the number that are found in winter.

While there is a relative increase in the number of bacteria during the winter months, there is also an increase in the volume of water, thereby dissipating in a measure the danger. This, I believe, is the explanation of why there is an increase in the number of cases of typhoid fever in the summer time over those of the winter. Now, if the river water would purify itself by means of aeration, light, and sedimentation, it should occur through the summer months, when the conditions are most favorable. We would expect a gradual diminution of sewage and colon groups of bacteria, and especially those contained in the Potomac River.

1

« السابقةمتابعة »